Some chemicals used in, for example, semiconductor device processing or other thin film fabrication processes, are delivered into a process chamber using a liquid containing ampoule. Such ampoules typically include a canister that is in fluid communication with an inlet port and an outlet port using a carrier gas that is fed through the ampoule. For example, the carrier gas is fed into the liquid using a bubbler tube, saturates the fluid, and carries away some of the liquid. As another example, the carrier gas is fed into a space above the liquid, may flow over the liquid, and carries away vapor from the space above the liquid. The ampoule may be used until all liquid is removed, at which time the ampoule is refilled or replaced. Alternatively, the ampoule may be refilled periodically based on measurements taken using an in situ level sensor. A refill tank is employed to hold chemicals for refilling the ampoule and typically has a much larger volume than the ampoule to enable the ampoule to be repeatedly refilled. However, the inventors have observed that the ampoule is typically refilled before the liquid is completely depleted, thus wasting an unused volume of liquid at the bottom of the ampoule. Therefore, the inventors believe that it would be advantageous to minimize the unused volume of liquid because the chemicals are expensive.
Accordingly, the inventors have provided improved apparatus for measuring the level of a liquid to be provided in a substrate process.